Nasty nasty nasty

Okay, so like my boyfriend is living with his oldest sister and his three other siblings. He's alway with his older and younger sister. His oldest is like 19 and his youngest is like 14. They both look a lot older. Anyway, he's always ditching me for them. And sometimes they like start whispering and all three of them are always all over each other. I swear they're having an affair bc every time he tries something new on me he says he learned it from one of his sisters or his mom! should I worry that he's like insest or we